CODEWITHSUNDEEP
c++multithreadingopenmp
Eines He
Eines He

Reputation: 187

Why OpenMP program runs only in one thread

I just tried OpenMP with a simple c program

test() {
   for(int i=0;i<100000000;i++);
}
main() {
    printf("Num of CPU: %d\n", omp_get_num_procs());
    #pragma omp parallel for num_threads(4)
    for(int i=0;i<100;i++) test();
}

Compiled with g++ -fopenmp. It can correctly print out that I have 4 CPUs, but all test functions are running at thread 0.

I tried to modify the OMP_NUM_THREADS. But it has no effect also.

I had everything the same as the online examples but why wouldn't I get it to work?

Upvotes: 8

Views: 20907

Answers (5)

abdelhamed
abdelhamed

Reputation: 11

first choose project _> properties -> c/c++ -> language -> open mp support -> choose yes and then you will find above conformance mode (make it no )

Upvotes: 1

hamed246
hamed246

Reputation: 31

I had this problem in visual studio and finally I understood that I had forgotten to enable Open MP support in visual studio. It didn't give me any error but executed the program just for one thread

Upvotes: 3

GBY
GBY

Reputation: 1100

I encountered the very same situation on my ubuntu desktop when I extends numpy module with C code. openmp only ran with one thread. I happened to remove libopenblas-base and install libatlas-base-dev.(to deal with numpy installation problem) Then multi-threading openmp came back:)

I have tested it on a ubuntu server with 64 cores and it works just as my desktop! I think this is because libopenblas conflicts with libraries like atlas.

Upvotes: 0

Tudor
Tudor

Reputation: 62439

Your problem is here:

#pragma omp parallel for num_thread(4) <---

The correct clause is num_threads(4), not num_thread(4). Incorrect openmp pragmas are ignored and so you ended up with a sequential program. :)

I'm surprised you didn't get a compiler warning, because I did.

Upvotes: 7

prathmesh.kallurkar
prathmesh.kallurkar

Reputation: 5686

use the function omp_set_num_threads(4) before calling the omp parallel section.

also, how do you determine the number of threads ?? embed your printfs in a critical section just to make sure everything is getting printed.

Upvotes: 0

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